This invention relates generally to a reserve power source synchronized with a primary power signal on a primary power line to switch a load to an auxiliary power signal when at least one of a plurality of power failure conditions is detected in the primary power signal. More particularly, but not by way of limitation, the present invention relates to a computer back-up power source which switches the computer, or parts thereof, from a conventional power line to a reserve power line and from the reserve power line back to the conventional power line in synchronization with zero-crossings of the conventional power signal.
Reserve power supplies are, as is well known in the art, needed for protecting computers when a primary power supply, such as is obtained through a conventional AC outlet, exhibits a problem which could adversely affect the operation of the computer. In particular, data can be lost when there is a problem with the primary power supply. To provide adequate protection to a computer, for example, there is the need to be able to detect various power failure conditions in the primary power signal so that the reserve power source can be switched to the computer upon the occurrence of one of such conditions. This detection and the switching needs to be done quickly to avoid an interruption in the power provided to the computer because an interruption of even a few milliseconds can in some cases be disastrous to the operation of the computer.
Various conditions within the primary power signal can indicate a power failure of the type for which switching to the reserve power source is required. A total loss of the signal and a sustained decrease in the voltage magnitude of the primary power signal are two conditions which should be monitored. These conditions are respectively referred to as a power outage (total loss of the voltage signal) and a power brown-out (a reduction of the voltage below a load-operational minimum). Another condition which needs to be monitored is that of an overvoltage wherein the primary power signal exceeds what the load can safely handle. This can occur by, for example, a sustained excessive voltage magnitude or a temporary, but very high excessive voltage.
In addition to simply detecting one or more of the foregoing conditions, it is also desirable that when a power outage is detected, the reserve power supply be instantaneously switched into the circuit for driving the load. This instantaneous switching is particularly critical when a computer is included in the load because a power outage immediately creates the potential for interrupting the computer sufficiently that data storage and other operational abilities can be lost. An excessive power condition or a power brown-out condition is not as critical as a power outage if the high voltage or low voltage condition is detected within one half cycle (i.e., within a few milliseconds) of the primary power signal because the computer equipment can better withstand an overvoltage or reduced voltage condition, rather than a total outage condition, for such a relatively longer period of time.
Regardless of the power failure condition that is detected, it is desirable that the switchover between the primary power line and the reserve power line be made in phase, or in synchronization, with the primary power signal so that out-of-phase switchover problems, such as transformer saturation, will not occur.
Although these needs have been known and reserve power sources have been proposed or manufactured which address at least some of these needs, I am not aware of a reserve power source which can meet all of these needs in the relatively simple and inexpensive manner as provided by the present invention.